1. Field of the Invention
The present invention relates to a focal plane shutter of the type in which the leading and the trailing shutter blades are composed of a plural number of blade segments arranged near the exposing aperture.
2. Related Background Art
With the improvement of the film sensitivity and the wish for new types of image representation in the art, there is an increasing desire for further increase of the shutter speed of the camera. In fact, there has already been realized such a high speed camera by which a shutter speed up to 1/8000 sec. is possible.
In principle, the prior art focal plane shutters have a structure comprising a base plate, arms rotatably connected to the base plate and blade segments consituting the leading and trailing shutter blades to be driven through the arms. The arms include main arms and follower arms. The main and follower arms are so rotatably connected as to form a parallel link mechanism. Every main arm has a pin-receiving hole in which a driving pin is engaged. Through, the driving pins, a driving mechanism can move the blade segments between the aperture opening position and the aperture closing position. In the aperture opening position, the blade segments are folded up outside the aperture. In the aperture closing position, the blade segments are expanded to cover the aperture.
In the prior art focal plane shutters as mentioned above, it is a common practice to use the same quality of material having the same plate thickness for both the main arm and the follower arm, taking into account the running balance of the arms.
As readily understood, in order to attain such a very high shutter speed of 1/8000 sec. or over, a further reduction of weight is absolutely necessary for the prior art focal plane shutter. Only the reduction of weight of the blade segments is insufficient for this purpose. The weight of the arms also must be reduced as much as possible. However, the weight reduction of main arms involves a difficult problem that the impact resistance of the main arms must be adequately maintained at the same. Thus, two conflicting problems of light weight and adequate impact resistance must be solved at the same time. For example, if the plate thickness of the main arm is decreased for the purpose of weight reduction only, there may be produced such main arm which is no longer resistant against the impact applied to it by a stopper or braking element. Since the shutter element is stopped by its collision against the braking element, the repeating impact will have the adverse effect to widen the pin-receiving hole which is the driving power transmission part of the arm. It is known in the art that such widening of the pin-receiving hole on the main arm brings about a decrease of the accuracy of the exposure time due to time lag. The time lag is caused by the fact that when the shutter is operated, the start of the main arms is delayed relative to the start of the driving pins in the widened pin-receiving holes.